1. Field of the Invention
The present invention relates to a producing method of a thin film magnetic tape and the thin film magnetic tape produced by applying an oblique evaporation process.
2. Description of the Related Art
Currently magnetic tapes applied to digital video recorders and audio tape recorders, especially, thin film magnetic tapes produced by applying an oblique evaporation process, are gaining attention in order to accomplish high density and reduced thickness of the tapes.
FIG. 5 is a plan view of a thin film magnetic tape producing apparatus of the prior art, which applies the oblique evaporation process. FIG. 6 is a perspective view of the thin film magnetic tape producing apparatus of FIG. 5, with parts broken away and in section, showing proximity of a cooling can roll.
A magnetic layer of the above mentioned thin film magnetic tape is generally formed in a thin film by the oblique evaporation process. As shown in FIG. 5, the thin film magnetic tape producing apparatus 1A of the prior art, which applies the oblique evaporation process maintains a vacuum condition inside a vacuum chamber 2 by a vacuum pump (not shown). Inside the vacuum chamber 2, there provided a set of film winding rolls 3A and 3B, a set of tape guide rolls 4A and 4B, and a cooling can roll 5 being rotatable freely. During ordinarily forming a film on a base film F, the base film F wound on the film winding roll (hereinafter referred to a supply roll) 3A runs through the tape guide roll 4A, the cooling can roll 5, and the tape guide roll 4B in a forward direction shown by an arrow S1 to the film winding roll (hereinafter referred to a take-up roll) 3B.
The base film F is made by a PET (polyethylene terephthalate) film having a thickness of approximately 6.4 xcexcm as a substrate for the thin film magnetic tape. A cooling apparatus (not shown) is installed inside the cooling can roll 5 so as to control deformation of the base film F due to increased temperature during a evaporation process.
A crucible 6, which is made from MgO (Magnesia) and formed in a box shape, is installed at a lower right hand corner from the cooling can roll 5 inside the vacuum chamber 2. A magnetic metal 7 such as Co is contained inside the crucible 6.
A piercing electron gun 8, which is an evaporation heat source to melt and evaporate the magnetic metal 7, is mounted on a right wall 2a of the vacuum chamber 2 with pointing at the crucible 6 located diagonally downward. The piercing electron gun 8 emits an electron beam 8a towards the magnetic metal 7 inside the crucible 6. The electron beam 8a melts the magnetic metal 7 and evaporates so as to coat a surface of the base film F, which is moving along the cooling can roll 5.
It is necessary to cover both edges of the base film F so as to prevent a magnetic metal vapor 7a, which evaporated from the crucible 6, from evaporating on the cooling can roll 5 while the base film F is running. Further, it is also necessary to control an incidence angle of evaporation of the magnetic metal vapor 7a such as evaporated Co (generally called oblique evaporation) with respect to a surface of the base film F due to the requirements for electromagnetic transducing characteristics when producing a thin film magnetic tape. In order to prevent deposits in inappropriate areas, an incidence angle controlling mask 9 is installed between the cooling can roll 5 and the crucible 6 as shown in FIG. 6.
A width of the base film F is narrower than a width of the cooling can roll 5 in this situation shown in FIG. 6. In order to prevent the magnetic metal vapor 7a from evaporating on or invading into the cooling can-roll 5 at proximity of both edges of the base film F, the incidence angle controlling mask 9 covers an area between an edge of the cooling can roll 5 and a few centimeter inwards an edge of the base film F. An opening 9a of the incidence angle controlling mask 9 is extremely small in order to control an incidence angle of the magnetic metal vapor 7a such as evaporated Co to the surface of the base film F and a growth angle of particles growing on the base film F.
Referring back to FIG. 5, an evaporation incidence angle of the above mentioned opening 9a of the incidence angle controlling mask 9 is an incidence angle of evaporating the magnetic metal vapor 7a such as evaporated Co on the base film F with respect to a line normal to the surface of the base film F being wraparound the cooling can roll 5. The evaporation incidence angle is set within a range of angle from a maximum incidence angle xcex8 max to a minimum incidence angle xcex8 min.
An oxygen gas injection pipe 10 is attached on an inner surface of the incidence angle controlling mask 9 with facing toward the cooling can roll 5 in a direction to the minimum incidence angle xcex8 min side. Oxygen gas O2 blows off through several holes provided on the oxygen gas injection pipe 10 towards the magnetic metal vapor 7a evaporated from the crucible 6.
The electron beam 8a emitted from the piercing electron gun 8 is controlled by a deflection magnet 11, which impresses a deflection magnetic field onto a trajectory of the electron beam 8a, and another deflection magnet 12, which is installed near the crucible 6. By scanning the electron beam 8a in the longitudinal direction of the crucible 6, the magnetic metal vapor 7a such as evaporated Co is thinly laminated on the surface of the base film F as a Coxe2x80x94CoO magnetic film in a lateral direction of the base film F. By laminating the Coxe2x80x94CoO Magnetic film on the base film F continuously in the longitudinal direction of the base film F, a long enough thin film magnetic tape is wound on the take-up roll 3B.
In a case of producing a thin film magnetic tape as mentioned above, a size of the opening 9a of the incidence angle controlling mask 9 is strictly limited. An efficiency of actual usage of the magnetic metal vapor 7a such as Co evaporated from the crucible 6 is only about 10 to 15% while almost all of the magnetic metal vapor 7a became unnecessary evaporation. In order to improve a usage efficiency of the magnetic metal vapor 7a by increasing a size of the opening 9a of the incidence angle controlling mask 9 even slightly, it is necessary to further improve magnetostatic characteristics.
With emergence of magnetoresistive heads such as a GMR (giant magnetoresistive) head and an MR (magnetoresistive) head, there exists a certain tendency to install such a magnetoresistive head into a digital video tape recorder. Therefore, an urgency to drastically decrease layer thickness of a magnetic layer of a thin film magnetic tape exists in order to improve a SN ratio of the thin film magnetic tape. However, there is existed a problem such that magnetostatic characteristics of a thin film magnetic tape is deteriorated if layer thickness of a magnetic layer of the thin film magnetic tape is decreased in accordance with a current method.
An idea of placing a CoO nonmagnetic underlayer underneath a magnetic layer of a thin film magnetic tape is suggested. This method will be explained in a [Comparative Example 2] section. With a deposit of Coxe2x80x94CoO magnetic layer on a top of growth particles (columns) of an isolated CoO nonmagnetic underlayer and isolation of growth particles (columns) of a Coxe2x80x94CoO magnetic layer in accordance with the growth particles (columns) of CoO nonmagnetic underlayer, dimishing magnetic interaction among Coxe2x80x94CoO magnetic layer particles prevents degradation of magnetostatic characteristics associated with an extremely thin Coxe2x80x94CoO magnetic layer.
When placing a nonmagnetic underlayer underneath a magnetic layer of a thin film magnetic tape, further isolation of a magnetic layer effectively reduces magnetic interaction among magnetic layer particles. Such a method to favorably deposit a nonmagnetic underlayer for this purpose, however, had not yet been discovered.
Accordingly, in consideration of the above-mentioned problems of the prior art, an object of the present invention is to provide a producing method of a thin film magnetic tape and the thin film magnetic tape, which exhibits excellent magnetostatic characteristics.
According to an aspect of the present invention, there provided a producing method of a thin film magnetic tape by using a producing apparatus provided with a vacuum chamber comprising: a pair of film winding rolls rotatable in a first and second directions being opposite to each other for winding a base film; a cooling can roll rotatable in the first and second directions for cooling the base film while forming; a crucible containing a magnetic metal being installed under the cooling can roll; a heat source for evaporating the magnetic metal; an incidence angle controlling mask installed between the cooling can roll and crucible for controlling a maximum and a minimum incidence angles of the magnetic metal vapor with respect to a normal line of the base film; and an oxygen gas injection device for injecting oxygen gas toward the magnetic metal vapor, wherein the base film is formed in a regular forming process by advancing the base film wound on one film winding roll of the pair of film winding rolls in the first direction toward another film winding roll along the cooling can roll, the producing method comprising steps of: a first forming process for depositing the magnetic metal vapor on the base film as a nonmagnetic underlayer by injecting a predetermined injection amount of the oxygen gas while advancing the base film wound on the other film winding roll in the second direction toward the film winding roll along the cooling can roll from the minimum incidence angle side to the maximum incidence angle side of the incidence angle controlling mask; and a second forming process for depositing the magnetic metal vapor over the nonmagnetic underlayer as a magnetic layer by injecting the oxygen gas in a smaller injection amount than in the first forming process while advancing the base film wound on the film winding roll in the first direction toward the other film winding roll along the cooling can roll from the maximum incidence angle side to the minimum incidence angle side of the incidence angle controlling mask.
According to another aspect of the present invention, there provided a thin film magnetic tape comprising at least a nonmagnetic underlayer and a magnetic layer in sequence on top of a base film, wherein a growing direction of growth particles (columns) of the nonmagnetic underlayer being and another growing direction of other growth particles (columns) of the magnetic layer deposited on a surface of the nonmagnetic underlayer is an opposite direction with respect to each other in a plane perpendicularly intersecting the surface of the base film.
Other object and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.